Optical apparatus for measuring objects having a rectilinear profile

ABSTRACT

Apparatus for measuring an object ( 4 ) with at least one rectilinear edge or profile ( 4   a,    4   b ) parallel to a given direction (y), comprising: a laser source ( 1 ) which emits radiation ( 2, 2   a ) which impinges on the said edge or profile; a first cylindrical converging lens ( 5 ) the directrices of which are parallel to the direction (y), disposed downstream of the object ( 4 ); a spatial filter ( 6 ) disposed in the focal plane (A) of the first converging lens ( 5 ); a second converging lens ( 7 ) disposed downstream of the spatial filter ( 6 ); and photosensors ( 8 ) disposed in the focal plant (F) of the second converging lens ( 7 ).

[0001] The present invention relates to optical apparatus for measuringobjects having a rectilinear profile, of the type defined in thepreamble to claim 1.

[0002] For a better understanding of the state of the art and itsinherent problems, conventional apparatus, illustrated in FIGS. 3 and 4of the attached drawings, which has been produced to study the behaviourof a generic two-dimensional shape in the laboratory will first bedescribed.

[0003] In FIG. 3 a He—Ne tube laser 1 delivers a beam 2 of collimatedcoherent light of uniform light density with a high degree of coherence.The beam 2 is expanded by a beam expander 3 having a pin-hole filter anddirected onto an opaque object 4 the geometric dimensions of which it isdesired to measure. By the diffraction principle described by Fresneland Fraunhofer, at the points of interaction of the light wavefront withthe opaque object 4, that is to say along its edge or outline, there areformed new wavefronts of spherical type the radial components of whichare divergent with respect to the direction of the original incidentbeam (FIG. 4).

[0004] At this level, if the image were to be analysed in detail, therewould be observed, in correspondence with the enlarged outline of theobject 4, a region of uncertainty created by thin “fringes” ofalternating light and shade which would render it difficult orimpossible to form an exact determination of the spatial position of theedge.

[0005] The light beam 2, after having encountered the object 4, arrivesat a spherical converging lens 5 disposed orthogonally of the directionof the beam.

[0006] According to the laws of geometric optics, only and exclusivelythe parallel components (indicated 2 a) of the incident radiation (FIG.4) converge at the focal point of the lens 5. At the focus of the lens 5is disposed an obstacle 6, such as, for example, an opaque spot,hereinafter called a “stop” or “spatial filter” which impedes thepropagation of the light.

[0007] The function of the spatial filter 6 is to stop only the parallelcomponents 2 a of the incident beam without interfering with thedivergent and diffracted components 2 b of the beam which can reach afocusing and enlarging converging lens 7 and finally be collected on ascreen or photo sensitive chamber 8. The resulting image on the screen8, after the spatial filtering just described, is constitutedexclusively by thin lines of light which correspond to the outline ofthe object 4 standing out on a dark background.

[0008] The contrast between the illuminated line (useful signal) and theresidual background illumination (noise) is greater the more thecoherent light source satisfies the initial requirements of spatialhomogeneity and parallelism.

[0009] The image thus processed lends itself particularly well toelectronic analysis for measurement of the object. In fact, whilst it isimpossible to establish exactly a criterion with which to choose areliable and repeatable preferential measurement point on an undefinedlight/shade edge (such as would be that of the image obtained withoutthe spatial filter), it is relatively simple to measure the distance, onthe screen 8, between spaced lines, each of which has a very narrowmaximum of luminous intensity.

[0010] Examples of theoretical studies which represent the knowntechnique discussed above are described in the following publications:H. Lipson and C. A. Taylor Fourier Transform and X-Ray diffraction,1958; G. Harnburn, C. A. Taylor, T. R. Welberry Atlas of OpticalTransforms, 1975; F. Docchio, E. Sardini, O. Svelto, A. Taroni On-LineDimensional Analysis of Surfaces Using Optical Filtering and ElaborationTechniques in the Fourier Plane, 1989; and R. G. Wilson Fourier Seriesand Optical Transform Techniques in Contemporary Optics, 1995.

[0011] However, the results achieved experimentally have been obtainedin a laboratory using sophisticated optical and electronicinstrumentation, in conditions very close to the theoretical ideal, thatis to say:

[0012] it was possible to have available a light source having a highdegree of coherence, typically that delivered by a He—Ne tube laser;

[0013] it was possible to filter as much as necessary, and expand thelaser beam in such a way that the resultant beam had a uniform luminousdistribution and the rays were parallel to one another; in other wordsit was possible to obtain a beam free from spatial harmonic componentsand having a flat wavefront;

[0014] it was possible to use high quality lenses and optics, with verylarge apertures with respect to the dimensions of the object underobservation.

[0015] Currently there are available on the market non-contactmeasurement devices (which utilise different principles from thosedescribed here) with which it is possible to obtain good measurementprecision (typically 0.1 μm) but at high cost and with limitedrobustness of the instrument, or else, alternatively, to obtain economybut limited precision (typically not less than 5 μm).

[0016] It is the object of the present invention to provide improvedmeasurement apparatus able to perform high precision measurements (0.5μm or less).

[0017] Another object of the invention is to provide measurementapparatus of low cost and small dimensions which make it suitable foruse in an industrial environment on a large scale. In particular, it isthe object of the invention to provide reliable measurement apparatus inwhich the maintenance operations are reduced to the minimum andadjustment is simple.

[0018] A further object of the invention is to provide measurementapparatus able to obtain continuous measurement of the object, with ameasurement limit better than that achieved with repeated scansions.

[0019] These and other objects and advantages, which will be betterunderstood hereinafter, are achieved according to the present inventionby apparatus having the characteristics defined in the attached claims.

[0020] A preferred but non-limitative embodiment of apparatus accordingto the present invention will now be described making reference to theattached drawings, in which:

[0021]FIG. 1 is a schematic side view of measurement apparatus accordingto the invention;

[0022]FIG. 2 is a schematic plan view of the apparatus of FIG. 1;

[0023]FIG. 3 is a schematic view of measurement apparatus of known type;and

[0024]FIG. 4 is an enlarged view of a detail of FIG. 3.

[0025] In the explanatory views of FIGS. 1 and 2 the dimensions,proportions and shapes of the objects and the angles of divergence orconvergence of the light rays are accentuated for clarity of explanationand are not shown to scale.

[0026] Making reference now to FIGS. 1 and 2, and utilising forsimplicity the same reference numerals already used in FIGS. 3 and 4 toindicate the same or corresponding parts and elements, the referencenumeral 4 indicates an object to be measured, in this example a wire thediameter of which it is desired to measure.

[0027] Naturally, the reference to this possible field of applicationmust not be interpreted in any way as limitative of the scope of thepatent. In particular, the apparatus according to the present inventionlends itself to obtaining:

[0028] measurement of the spatial position of a rectilinear edge (oroutline or section) of a mechanical object, referred to a zero of theapparatus, or

[0029] the relative distance between two or more rectilinear paralleledges of an object.

[0030] The invention can therefore be utilised for the measurement of athread, yarn or wire, or a turned mechanical piece, or the position ofthe rectilinear edge of a sheet of opaque material (sheet metal,plastics laminate, paper etc.) or a rectilinear slot having paralleledges (the space between two pieces of sheet metal about to be welded)etc.

[0031] Given that the particular type of object which it is intended tomeasure is known a priori it is possible to introduce importantsimplifications to the optical structure designed for the extraction ofthe outline of the object, making the process less sensitive to possibleinhomogeneities or disturbances, and applicable at the industrial level.

[0032] In FIGS. 1 and 2 is indicated a set of three orthogonal axes (x,y, z); the object 4 to be measured is disposed with its rectilinearparallel edges 4 a, 4 b orientated along the y direction, defined hereas the longitudinal direction, for the purpose of measuring the diameter(or the spatial position) of the object 4 along the vertical x axis.

[0033] A light source 1 projects a collimated light beam 2 in thedirection of the axis z, defined here as the transverse axis. The beam 2is expanded by an expander 3 which does not have filters but includes apair of lenses 3 a, 3 b to obtain an expanded light beam 2 a orientatedparallel to the transverse z axis and of width such as to embrace boththe upper and lower outlines 4 a, 4 b respectively of the wire 4.

[0034] The expanded light beam 2 a illuminates the wire 4 and aconvergent lens 5 disposed downstream of this.

[0035] An important characteristic of the present invention is that theconvergent lens 5 is of cylindrical type with its generatrices disposedparallel to the y axis. The lens 5, being of cylindrical type, producesnot a point focus but a rectilinear one parallel to the longitudinal yaxis. In the focal plane A of the lens 5 is disposed a spatial filter 6of essentially linear form elongated in a direction parallel to thelongitudinal y axis.

[0036] The spatial filter 6 may advantageously be constituted by a baror by a calibrated wire the width of which in the x dimension is chosenin dependence on the specific requirements to obtain a good compromisebetween the capacity for obtaining a sufficiently well defined radiationwhich passes through the spatial filter and at the same time having alight intensity sufficient for the purpose of detection. This spatialfilter can be advantageously fixed in a simple manner at its oppositeends to the apparatus. The spatial filter 6 can be made of any material;at least the surface 6 a of the spatial filter facing the cylindricallens 5 is opaque. For example the spatial filter 6 can be constituted bya black or blackened metal wire.

[0037] Preferably the wire 4 is situated at the front focal distance ofthe lens 5.

[0038] The adoption of a cylindrical converging lens 5 in place of aconventional spherical lens is advantageous in that it makes it possibleto introduce significant improvements of a practical nature to theoptical process of extraction of the outline of an image. The wave frontof a collimated and expanded light beam 2 a which illuminates the wire 4is considered. Without wishing to be tied to any specific theory, theexperiments conducted by the applicant show that, with respect to theconventional arrangements with spherical converging lenses, in theoptical process of “extraction” of the outline of the object 4, thefinal image obtained with a cylindrical lens is less sensitive topossible inhomogineities present in the illumination wavefront, in thatpossible inhomogineities in the distribution of light along thelongitudinal y axis, not being subjected to the effect of the lens 5 inthe xz plane, do not increase the residual background illumination. Thiseliminates an important optical noise factor.

[0039] Therefore it is possible to use a source 1 of collimated lightwhich is not ideal, such as a laser diode 1 of low power, for exampleabout 3 mW, which typically has a non-uniform spatial light emission.Better results are obtained if the axis of polarisation of the electricfield of the luminous radiation emanating from the laser diode 1 isorientated along the vertical x axis of the lens; in these conditionsthe minimum residual background noise and the maximisation of the usefulsignal is obtained.

[0040] From an industrial point of view it will be appreciated that theuse of a laser diode in place of a tube laser source, as proposed in thecited publication of F. Docchio et al, drastically reduces the totalcost and the overall dimensions of the apparatus.

[0041] It has been experimentally verified that due to the use of acylindrical lens 5 the resultant signal-to-noise ratio in the extractedimage is excellent even using a non-ideal light source (low power laserdiode) and without using filters or particularly dedicated optics torender the light emission uniform.

[0042] Moreover, a spherical lens normally uses a point-like stop whichmakes it necessary for it to be mounted on a transparent support. Thisinvolves the disadvantages of a greater cost of production, thepossibility of optical aberrations caused by the transparent support,possible accumulations of dirt on the transparent support, as well asthe difficulty of centring the point-type spatial filter at the focalpoint of the spherical lens.

[0043] The cylindrical lens 5 makes it possible to effect enlargement ofthe image only along the x axis on which it is desired to effect themeasurement. Along the longitudinal y axis it is instead preferable tobe able to have available a wide field of view downstream of the lens 5(see FIG. 2) in such a way as to have available the greatest possiblenumber of sections of the object in the planes parallel to the xz planeon which to be able to effect the greatest possible number of individualsimultaneous measurements. The cylindrical lens achieves this condition,whilst a spherical lens inevitably would enlarge the image in both the xand y directions making possible repeated measurements of the object 4limited to a section of very modest length.

[0044] The light rays 2 c, 2 d which pass the spatial filter 6 arefocused on a photosensitive electronic device 8 disposed in the focalplane B of a further converging lens 7 in such a way that the frontfocus of this coincides with the rear focus of the cylindrical lens 5 asis known from the geometrical laws of optics. The resultant enlargementis equal to the ratio of the focal lengths of the two lenses.

[0045] The light rays which are not deviated along the y direction (FIG.4) in the passage through the cylindrical lens 5 converge on the focalplane F of the spherical lens 7 on the photo sensitive screen 8. In theexample of the wire 4 illustrated in FIGS. 1 and 2, each of the twolight beams 2 c and 2 d which pass the spatial filter 6 are caused toconverge at a respective point C, D on the photo sensitive device 8.

[0046] Owing to the adoption of a cylindrical lens 5 the resultant imagefocused on the photosensitive device 8 will, in general, be constitutedby n light spots, each corresponding to one of the n rectilinear edgesof the object 4, and all aligned along the x axis. The relativedistances between the spots C, D are proportional to the real dimensionsof the object.

[0047] It is important to note that, by virtue of the configuration ofthe apparatus according to the invention, all the information relatingto the whole segment of the edge—extending along the y axis—within thefield of view of the apparatus converges contemporaneously andinstantaneously in each spot focused on the photosensitive device 8.

[0048] In reality the spatial distribution of the luminous intensitydetectable in each spot is not a point, but of gaussian type in that itis the integral sum of the individual infinitesimal contributions comingfrom each point belonging to an edge of the object 4, taken along the yaxis across the field of view.

[0049] The intensity and extent of this gaussian distribution isindicative of the degree of uniformity of the contribution provided bythe various points of a section of the edge of the object 4 which isbeing measured. These parameters detect if in the section underobservation there are defects or irregularities in the outline of theobject 4.

[0050] The apparatus according to the invention can conveniently beutilised to effect measurements on an object in motion, for examplemeasuring the diameter of a wire 4 which advances along the longitudinaly axis. It should be noted that the total time used by a suitableelectronic unit for the conversion of the light image into signals(digital or analogue) and for processing them, although short, is notzero. To ensure that the measurement of an object 4 in movement alongthe Y axis is continuous, or rather that all the points of the profileof the object have been processed, excluding the possibility ofnon-monitored zones, it is sufficient to obtain that the visual field ofthe apparatus be equal at least to the distance travelled by the object4 at its maximum velocity in the overall time taken for conversion andprocessing of the image.

[0051] Due to the simplifications introduced into the optical structureadopted by this invention it is sufficient to use, as photosensitivedevice 8, an electronic transducer of linear type in that all theinformation necessary to extract the desired measurements is containedin a series of spots disposed rigorously along a straight line along thex axis. The electronic transducer 8 is conveniently mounted in a fixedmanner on the apparatus.

[0052] If the object 4 to be measured is such as to produce on thephotosensitive device 8 a single spot (for example in the case of ametal sheet the position of a single edge of which is to be measuredwith precision), a linear photodiode of PSD (Position Sensing Device)type can be utilised with exceptional results of precision and speed ofresponse.

[0053] If, on the other hand, there are two spots (for example to obtainthe measurement of the diameter of a wire), or more than two, it ispossible to use a CCD (Charge Couple Device) of linear type, able toprovide an electrical signal which faithfully represents the spatialdistribution of the light image incident on its sensitive surface.

[0054] It is intended that the invention be not limited to theembodiments described and illustrated here, which is to be considered asan example of the measurement apparatus of the invention. The inventionis, on the other hand, susceptible of modifications relating to form,disposition and number of components of the apparatus, as well as theconstructional and functional details.

1. Apparatus for measuring an object (4) having at least one essentiallyrectilinear outline or edge (4 a, 4 b), parallel to a given direction(y), the apparatus comprising: a laser source (1) capable of emittingradiation (2, 2 a) which impinges on the said at least one edge oroutline (4 a, 4 b); first converging lens means (5) disposed downstreamof the object (4) and comprising at least one cylindrical lens (5) withits directrices parallel to the said given direction (y), spatial filtermeans (6) disposed in the focal plane (A) of the first converging lensmeans (5); second converging lens means (7) disposed downstream of thespatial filter means (6); photosensitive means (8) disposed in the focalplane (F) of the second converging lens means (7); characterised in thatthe spatial filter means (6) comprise an element essentially in the formof a wire or bar elongated in a direction parallel to the said givendirection (y).
 2. Apparatus according to claim 1, characterised in thatthe said laser source (1) comprises a laser diode.
 3. Apparatusaccording to claim 2, characterised in that the said laser diode isadapted to emit radiation (2) polarised in planes (xz) perpendicular tothe said given direction (y).
 4. Apparatus according to claim 3,characterised in that the said laser diode is a low power laser diode.5. Apparatus according to claim 1, characterised in that the saidphotosensitive means (8) comprise at least one linear photodiode of PSDtype orientated along a direction (x) perpendicular to the said givendirection (y).
 6. Apparatus according to claim 1, characterised in thatthe said photosensitive means (8) comprise at least one linear CCDtransducer orientated along a direction (x) perpendicular to the saidgiven direction (y).